The present invention relates generally to the field of magnetic resonance imaging systems, such as those used for medical diagnostic applications. More particularly, the invention relates to a technique for shielding gradient coils in magnetic resonance imaging systems from radiofrequency magnetic fields generated during the course of examinations.
Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. Over the two past decades, improved techniques for MRI examinations have been developed that now permit very high-quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.
In general, MRI examinations are based on the interactions among a primary magnetic field, a radiofrequency (rf) magnetic field and time varying magnetic gradient fields with nuclear spins within the subject of interest. Specific nuclear components, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of such nuclear components can be influenced by manipulation of the fields to obtain rf signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to produce images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of three gradient coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. A radiofrequency coil is employed to produce an rf magnetic field. This rf magnetic field perturbs the spin system from its equilibrium direction, causing the spins to precess around the axis of their equilibrium magnetization. During this precession, radiofrequency fields are emitted by the spins and detected by either the same transmitting rf coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to reconstruct a useful image.
Many specific techniques have been developed to acquire MR images for a variety of applications. One major difference among these techniques is in the way gradient pulses and rf pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios, and resolutions. Graphically, such techniques are illustrated as xe2x80x9cpulse sequencesxe2x80x9d in which the pulses are represented along with temporal relationships among them. In recent years, pulse sequences have been developed which permit extremely rapid acquisition of a large amount of raw data. Such pulse sequences permit significant reduction in the time required to perform the examinations. Time reductions are particularly important for acquiring high resolution images, as well as for suppressing motion effects and reducing the discomfort of patients in the examination process.
While field interactions are fundamental to the encoding of data acquired in MRI systems, certain field interactions are undesirable, or may lead to degradation of the image data. For example, when the appropriate pulses are applied to an rf coil during an examination sequence, rf energy from the rf coil can penetrate the gradient coil structure where it is dissipated by lossy eddy currents induced in the gradient coil structure. To maintain a high efficiency of the rf coil, then, an rf shield is typically positioned between the rf coil and the gradient coil set so as to prevent or reduce penetration of the rf magnetic field into all of the gradient coils. The design of the rf shield is such that minimal eddy currents are generated by switching of the gradient fields, rendering the rf shield substantially transparent to the gradient fields. At the same time, the rf frequencies are much higher than characteristic eddy current decay rates in the shield, hence the shield functions like an impenetrable barrier to rf fields.
The proximity of an rf shield to the gradient coil conductors, particularly in the case of a whole body rf transmit coil, may significantly affect the overall power efficiency and the signal-to-noise ratio of the rf coil. In general, it is desirable to place the rf shield as far as possible from the rf coil. Where the power efficiency is reduced, larger amounts of power may need to be supplied to the rf coil, leading to the use of larger power amplifiers to obtain a desired magnitude of the rf magnetic field. Larger currents may also be required for the rf coil conductors, potentially leading to unacceptably high levels of energy within the patient bore. Moreover, coupling with the shield effectively increases the series resistance of the rf coil and lowers the inductance. These combined effects may result in a low quality factor (sometimes referred to at xe2x80x9cQxe2x80x9d in the art), and a consequent reduction in signal to noise ratio.
There is a need, therefore, for an improved technique for shielding rf magnetic fields in MRI systems. There is, at present, a particular need for a technique which can be employed in a straightforward manner to enhance both the power efficiency of the rf coil and the signal-to-noise ratio to address the drawbacks in hereto for known systems.
The present invention provides a radiofrequency shielding technique designed to respond to such needs. The technique may be employed in a wide range of systems, but is particularly suitable to magnetic resonance imaging systems, such as those used in medical diagnostic applications. The technique may also be employed in any suitable MRI scanner design, including full body scanners, open scanners, and scanners with a range of field ratings. Where appropriate, the technique may be used to retrofit existing scanners, or may be incorporated into new designs, particularly in the configuration of the gradient coil structure.
The technique makes use of a novel arrangement of gradient coils and an rf shield. In one embodiment, the rf shield is placed between the gradient coils, with a modified solenoid-type coil, commonly the Z-axis gradient coil, positioned within the shield, that is, between the shield and the rf transmit coil. Because the mode of the rf coil that is typically used in MRI has little or no net magnetic flux in the Z-axis direction, the coupling between the rf coil and the Z-axis gradient coil is minimal. Hence the radiofrequency field will be disturbed very little by the presence of the Z-axis gradient coil on the interior of the shield surface, enabling the rf shield to be moved significantly away from the transmit coil as compared to known designs. The technique has been demonstrated to provide a significant reduction in noise and increased efficiency, allowing for use of a smaller rf amplifier than in conventional systems, or for reduced power input to the rf transmit coil to obtain the desired rf magnetic field strength.